The present invention relates generally to analog optical information systems, and more specifically the invention pertains to a system which enhances the dynamic range of spatial light modulators used with analog optical processing systems, and optical data systems with linear arrays of numerical data.
In analog optical computing systems light is the information carrying medium. For such systems sufficient dynamic range is needed (ie., a number of resolvable information levels, rather than just the information of "on" and "off"). The purpose of this invention is to provide improved dynamic range in one dimension, so that a one dimensional array of nonnegative numbers can be accurately represented as a linear spatial light pattern. This spatial light representation of numbers can then be used as input to other optical computing components, such as acousto-optic cells or linear detector arrays. The spatial light pattern can remain fixed for an indefinite period of time, or can be dynamically changed many times per second. This is an electronically addressed device, so it also acts as an electronic-to-optic interface which can be used to accurately introduce one dimensional data into the optical realm from a digital electronic source.
Previous attempts to incorporate one dimensional spatial light modulation in analog optical computing architectures have taken one of four different approaches: use a one dimensional cross section of a 2-D spatial light modulator (SLM), essentially ignoring the second dimension; use a linear array of laser diodes or light emitting diodes (LEDs); use an acousto-optic cell; and use a fixed mask such as photographic film negative.
Each of these approaches has disadvantages. The 2-D SLM approach is probably the most common. Existing 2-D SLMs have been developed primarily for image processing applications, where a high degree of numerical accuracy is not needed. These devices are typically capable of representing between 2 and a maximum of 10 levels of numerical resolution (ie., a maximum of one decimal place of numerical accuracy). One dimensional spatial light modulation is achieved with these devices simply by considering only a linear cross section of the two dimensional output, thus effectively ignoring the second dimension. This one dimensional spatial light modulation thus suffers from the same low dynamic range as the two dimensional output of the device.
Electrically addressed 2-D SLMs include the Semetex SIGHT-MOD (magneto-optic spatial light modulator), the Texas Instruments Deformable Mirror Device, the Displaytech Ferroelectric Liquid Crystal (FLC) Display, and commercial light crystal television displays, such as those made by Radio Shack. Hughes makes the Liquid Crystal Light Valve, however this device is optically addressed, and so cannot serve as an electronic-to-optic interface.
Arrays of laser diodes or LEDs is another frequently proposed approach to 1-D spatial light modulation. If a large number of components in the linear array is desired (such as the 100 or more that our approach can provide), then the laser diode approach would be prohibitively expensive. LEDs are more economically feasible, but one must be concerned with the following potential drawbacks: incoherent light source (a problem if acousto-optic cells are to be used later in the system), nonuniformity among the LEDs, possible nonlinear response over input range, and low dynamic range. Also, physical spacing between LEDs or laser diodes may cause problems in some applications.
Acousto-optic cells can also be considered as one dimensional spatial light modulators. Modulation is produced by continually feeding an electronic signal into a transducer. This sends an acoustic wave through the crystal material of the device, producing a moving pattern of altered indices of refraction in the crystal. This moving pattern acts as a diffraction grating that modulates the intensity of the first diffracted order of the transmitted light. This type of modulation differs from the optical information modulation of the present concern in that it requires a continuous input of signal data, and it produces a continually moving spatial light pattern. It is not suitable for representing an array of numbers which can be either fixed or dynamically changing.
Fixed masks have been used for laboratory demonstrations that require one dimensional spatial light modulation. The problem with this approach is the obvious one that the modulation is fixed, and cannot be changed in real time.
The task for providing extended dynamic range for one dimensional spatial light modulation system is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are specifically incorporated herein by reference;
U.S. Pat. No. 4,813,761 issued to Davis;
U.S. Pat. No. 4,815,799 issued to Goldstein; and
U.S. Pat. No. 4,867,543 issued to Bennion.
The patent to Davis (761) teaches high efficiency programmable diffraction gratings using a spatial light modulator. The patent to Goldstein (799) teaches a spatial light modulator responsive to infrared radiation. The patent to Bennion (543) teaches a spatial light modulator employing a solid ceramic material having high electro-optic coefficients.
While the prior art systems are instructive, a need remains to extend the dynamic range for one dimensional spatial light modulation information carrying systems. The present invention is intended to satisfy that need.